Magneto inductive flow measuring devices utilize for volumetric flow measurement the principle of electrodynamic induction: Charge carriers of the medium moved perpendicularly to a magnetic field induce a voltage in electrodes likewise arranged essentially perpendicular to the flow direction of the medium and perpendicular to the direction of the magnetic field. The induced measurement voltage is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube and, thus, proportional to the volume flow rate. If the density of the medium is known, the mass flow in the pipeline, respectively the measuring tube, can be determined. The measurement voltage is usually tapped via the electrode pair arranged in the region of the measuring tube where the maximum magnetic field strength and, thus, the maximum measurement voltage is to be expected. The electrodes are usually coupled galvanically with the medium; however, also known are magneto inductive flow measuring devices with contactless, capacitively coupling electrodes.
The measuring tube can be manufactured either from an electrically conductive material, e.g. stainless steel, or from an electrically insulating material. If the measuring tube is manufactured from an electrically conductive material, then it must be lined in the region coming in contact with the medium with a liner of an electrically insulating material. The liner is composed, depending on temperature and medium, for example, of a thermoplastic, thermosetting or elastomeric, synthetic material. However, also known are magneto inductive flow measuring devices having a ceramic lining.
Historically, such two-conductor field devices are predominantly so designed that an instantaneous electrical current level of the supply current instantaneously flowing in the single pair line formed as an electrical current loop is set at a value lying between 4 mA and 20 mA and simultaneously also represents the measured value instantaneously produced by the field device, respectively the actuating value sent instantaneously to the field device. As a result of this, a special problem of such two-conductor field devices is that the electrical power at least nominally usable or to be used by the field device electronics—in the following referred to for short as “available power”—can during operation fluctuate in a practically unpredictable manner over a broad range. Taking this into consideration, modern two-conductor field devices (2L field devices), especially modern two-conductor measuring devices (2L measuring devices), with a (4 mA to 20 mA) electrical current loop are, consequently, usually so designed that their device functionality implemented by means of a microcomputer provided in the evaluating and operating circuit is changeable, and, thus, the operating and evaluating circuit, which most often in any event uses only little power, can be matched to the instantaneously available power.
Of all the components of a magneto inductive flow measuring device, the coil arrangement for producing the magnetic field B passing through the measured material has the greatest energy requirement.